Sect. I. KEELING ATOLL. 1 3 



second in stony knobs, as thick as a man's finger, 

 radiating from a common centre ; and the third, which 

 is less common, in a moss-like reticulation of thin, but 

 perfectly rigid branches. 1 The three species occur 

 either separately or mingled together ;' and they form 

 by their successive growth a layer two or three feet in 

 thickness, which in some cases is hard, but where formed 

 of the lichen-like kind, readily yields an impression to 

 the hammer : the surface is of a reddish colour. These 

 jSTulliporsB, although able to exist above the limit of 

 true corals, seem to require to be bathed during the 

 greater part of each tide by breaking water, for they are 

 not found in any abundance in the protected hollows on 

 the back part of the reef, where they might be immersed 

 either during the whole or an equal proportional time 

 of each tide. It is remarkable that organic productions 

 of such extreme simplicity, for the Nulliporse undoubt- 

 edly belong to one of the lowest classes of the vegetable 

 kingdom, should be limited to a zone so peculiarly cir- 

 cumstanced. Hence the layer composed by their growth, 

 merely fringes the reef for a space of about 20 yards in 

 width, either under the form of separate mammillatecl 

 projections, where the outer masses of coral are separate, 

 or more commonly, where the corals are united into a 

 solid margin, as a continuous smooth convex mound 



1 This last species is of a beautiful bright peach-blossom colour. Its 

 branches are about as thick as crow-quills ; they are slightly flattened 

 and knobbed at the extremities. The extremities only are alive and 

 brightly coloured. The two other species are of a dirty purplish white. 

 The second species is extremely hard ; its short knob-like branches are 

 cylindrical, and do not grow thicker at their extremities. 



